Up the Milky Way

Note - we might have this discussion any time during
the sky tour, whenever it comes up.

You can see the Milky Way on any clear summer night, but up in the north
woods, on especially clear nights, the Milky Way glows like a neon light,
from north horizon to south horizon, brilliantly reflected in the lake by
our camp. Most people recognize the Milky Way as a band of light across
the night sky, but what is that band of light - where does it come from?

Well it's a bit like looking at a gravel road. Close to you, you can see
individual stones, while off in the distance the road merges into a uniform
gray. When you look at the Milky Way you are looking at the disk of our
galaxy from the inside - the nearby stars you can see individually (most
are less than 1,000 light years away), the distant stars merge into a hazy
glow. So when you see that band across the sky you are really looking way
off into the distance, into the "billions and billions" of stars that make
up the disk of our galaxy.

You'll see a similar phenomenon later when you look at the
M13 globular cluster in
Hercules. Through binoculars you will see it as a
hazy patch. When you see it through a telescope - that is, closer up - you
can pick out individual stars. The better the telescope, the 'closer' you
can get to M13, the more stars you can see.

We can find some more constellations -- some of the best -- by following the
Milky Way up from Scorpius and Sagittarius. First we come to Aquila the Eagle.
{Trace out Aquila.} The bright star is Altair, and the two stars on either
side of Altair are called the Pilot Stars. Altair is 16 light years away.
It's about half again as big as our sun and nine times as bright. That shows
how sensitive stars are to mass -- just a little more mass causes the star to
burn much brighter.

The star that is about halfway between Aquila's eastern wingtip and his
body is called h (Eta) Aquilae -- and this is another
Cepheid variable with a cycle of about
7 days. At its brightest it's nearly as bright as the body star,
d (Delta) Aquilae (actually it's as bright as the
eastern Pilot Star b Aquilae), and at its dimmest
it's about as bright as i (Iota) Aquilae.
This is one of only three Cepheids you can see by eye - you already met
d Cephei, the original, and the third one is
in the winter constellation of Gemini.

If you look with the binoculars you can find the two stars of Aquila's tail,
then slide west (to the right) until they are at the far left of the field
of view. To the far right of your field of view you can now see a fuzzy
spot with a star at the center - you have found the Wild Duck Cluster. In
a telescope this looks like a dense cluster of tiny stars broken into groups -
the central group is V-shaped and looked to early observers like a flight of
wild ducks - hence the name.

The next constellation we reach is Cygnus the Swan. {Trace out Cygnus}. This
constellation is also known as the Northern Cross and at Christmas time, in the
evening, it has rotated across the sky so it is standing on its base on the
northwestern horizon, with the bright star Vega next to it. Very Christmas-y.

Deneb is the star at the tail of the Swan. Notice that it is about as bright
as Altair, which is 16 light years away -- so how far would you guess Deneb is?
As it happens... Deneb is 2600 light years away - about 150 times farther!!
Deneb is actually 20,000 times brighter than Altair and 200,000 times brighter
than our sun. If Deneb were as close as Altair, it would be visible in broad
daylight and would cast a shadow at night. This star is one of the greatest
super-giant stars known.

Notice in the picture above that Altair and Deneb are two of the three
bright stars overhead, which form the "Summer Triangle". You will be introduced
to the third (Vega) in just a moment.

Albireo, the star at the head of the swan, is actually a beautiful double
star, a yellow giant and a blue main-sequence star. You can just split this star
with good binoculars held steady, and it looks better in a telescope.

The Milky Way looks like a single solid band as you follow it from Cassiopeia
through Cygnus to Sagittarius, but this is misleading. When you look at the
Great Star Cloud in Sagittarius, you are looking toward the center of the
galaxy but you are not actually seeing the center of the galaxy. You
are actually looking at the great, massive spiral arm next to ours, the next
one in toward the center. There are huge lanes of dust and another spiral arm
before you finally reach the center. When you look at Cygnus, you are looking
right down the beam of our local spiral arm, which is really just a tiny branch
off the Sagittarius arm. Then when you look at Cassiopeia, you are looking at
the great massive spiral arm that just outside ours. Beyond that arm you are
outside our galaxy.

Just above and to the east of Aquila you can spot a small diamond of stars with
a little tail coming down. This is the tiny constellation, Delphinus the Dolphin.
It really looks like a little dolphin, doesn't it?

Then next to Delphinus, continuing to the left (east) is another teeny tiny,
and faint constellation, Equuleus the Horse. It's four stars forming a horse's
head, with the nose pointing up, to the North. See it? Actually it's five
stars, the corner by his jaw is a pair of stars. Equuleus is nearly the
smallest constellation in the sky -- only the Southern Cross, next to the South
Pole, is smaller.

The two birds, Aquila the Eagle and Cygnus the Swan, are flying straight at each
other and, to enhance the suspense, Sagitta the Arrow flies between them, just
missing both. {Trace out Sagitta.} What is most interesting about Sagitta,
though, is not so much the constellation as the features for which it is a milestone.
(Note: these two features, technically, are in the neighboring and very faint
constellation "Vulpecula" the Fox.)

If you locate the two tail-feather stars of Sagitta in binoculars then follow
the line between them up (to the northwest), just one field of view, you will see
the Coathanger, an interesting cluster of stars.

If you are really good with those binoculars you can go up from the tip of the
arrow, toward Albireo in Cygnus, and you will find a group of stars that just fill
the field of view in the shape of an "M". Right at the central point of the "M" is
a faint fuzzy patch that actually is known as the "Dumbbell Nebula". To really see
its hourglass shape requires a telescope. This hazy patch once was a star, until it
blew itself apart.

This is called a "planetary nebula", because the typical disk shape (this
one is less typical) suggested the look of a planet to early astronomers. In
fact it has nothing to do with planets at all. This is what's left of a red
giant star that finally did what all red giants eventually do. When the fuel
at the core runs so low that the nuclear reactions can no longer hold up the
weight of the star, it all collapses in to the center, which in turn raises the
temperature so high that the star blows off its outer envelope of gases, losing
much of its mass. This exposes the core to outer space, or, more accurately,
exposes outer space to the nuclear reactions going on at the core. The
intense radiation from the burning core causes the expanding shell of gas to
light up like a neon light, and voila -- the faintly glowing dumbbell that you
see here. The burning core of this star is now a "blue dwarf" -- on some
planetaries you can see the star at the center. The Dumbbell's central star is
notoriously elusive, though, and we probably can't see it in our telescope.

So diffuse nebulae, like the two we saw in Sagittarius, are the birthplaces of
many new stars, and a planetary nebula is the deathbed of a single dying star. We
will see another, even more famous, planetary nebula in the next constellation.